<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        .son{
            width: 200px;
            height: 200px;
            background: red;
            float: left;
        }
        .father{
            width: 100px;
            /*height: 200px;*/
            background: green;
            overflow: hidden;
        }
        .test{
            width: 0;
            height: 0;
    border:10px solid transparent;
    border-right-color: #ddd;
        }
    </style>
</head>
<body>
<!--     <div class="father">
        <div class="son"></div>
    </div> -->
    <div class="test">
        
    </div>
</body>
</html>